Symposium EN03 : Superconducting Materials—From Basic Science to Applications

Since the discovery of high temperature superconductivity (HTS) the most advanced materials of top performance in this field are so-called superconducting tapes of the second generation (2G-HTS) – the new class of 2D nanomaterials consisting of several multilayers with precisely controlled chemical composition, structure and grain orientation within the scale of hundreds meters length. Manufacturing of long-length 2G-HTS tapes is extremely challenging from the materials point of view. Combination of most advanced sputtering techniques including ion-beam assisted deposition (IBAD) and pulsed laser deposition (PLD) methods has proved to be one of the most reliable and promising technique.
At present several companies including Fujikura, Bruker, Shanghai Superconductors and SuperOx routinely produce tens of kilometers of high performance 2G HTS tapes, however further prospects for up-scaling IBAD/PLD approach towards manufacturing of hundreds and thousands of kilometers of tapes is not clear yet. It depends on the several factors including overall situation on the superconducting market, which closely related with the price of cooling equipment and superconducting materials. In this presentation I am are considering IBAD/PLD upscaling from the viewpoint of recent progress in sputtering and laser technologies and the latest results in 2G wire production and designing of deposition equipment made in past 5 years by our company.
Since 2012 SuperOx group produced and delivered several tens of kilometers of superconducting tapes with high degree of customization for R&D and commercial projects worldwide. I will give a brief overview of main results obtained during past few years since the establishment of SuperOx Japan and describe our company strategy and vision on future development of 2G HTS wires production and applications.
In 2016 we made a substantial progress in up-scaling of our production and installation of independent new high-throughput equipment in Japan and Russia. Currently SuperOx activity for upgrading of manufacturing facilities aimed to increase the production throughput and yield and to improve the performance of 2G HTS wires with a wider range of customization of our product tapes. Based on our recent cost analysis model I will discuss the potential pathways to reduce the final price of our wires, which based on the magnetron-IBAD-PLD production scheme and discuss the main problems and obstacles of this approach in comparison with other techniques.
SuperOx involved in several R&D and commercial projects including development of advanced superconducting fault current limiters, cables, current leads, induction heaters, motors etc. In conclusion, I will present a brief overview of our ongoing and prospective projects aimed on the development of new materials, wire architectures, construction of pilot scale devices and electrical equipment.

SuNAM has been producing long-length coated conductors based on a proprietary process which consists of electron beam co-evaporation of constituent metals and subsequent conversion of precursor film to superconducting phase by carefully controlling temperature and oxygen pressure. After securing stable manufacturing routine for upto 1 km-long wires, with about 10 percent uniformity in critical current enabled by various quality control measures, we tried to further increase critical current above 1,000 A/cm-width. This goal should be achieved by increasing thickness of superconducting layer while retaining critical current density, or even enhancing it. We varied co-evaporation process to enhance composition uniformity, and also modify temperature and pressure profile in heat treatment furnace to better utilize conversion dynamics of GdBa2Cu3O7-x phase formation, and the results will be presented.
With these wire, we made 400 mH compact reactor with cryogen free operation. The reactor’s operating current is over 1,500 A at temperature is around 10 K. Detailed design, construction, and operating results will be discussed. And first all HTS CC base commercial 18 T magnet result will be introduced. A 70 mm cold bore high temperature superconducting (HTS) magnet was developed for axion detector system of Center for Axion and Precision Physics (CAPP) research center in Institute for Basic Science (IBS) in the Republic of Korea. A key parameter for axion detector magnet is to generate high and longitudinally uniform magnetic field in RF cavity. Magnetic field strength on -100 mm < z < 100 mm in coil bore space should be larger than 90 % of it at magnet center.
Finally we’ll summarize a commercialization & industrialization efforts in Korea, and suggest a key issues to open the true market.

During the past 25 years, worldwide R&D efforts were concentrated on development of REBCO coated conductor (C.C.), which was expected to have the strongest pinning properties in wide temperature/field range. The in-field performance for REBCO conductors still have growth potential by using artificial pinning centers (APC), as BMO doping technology with nanometer size rod-like structure, etc. But it was a tough challenge to get end-to-end uniformity even for non-APC REBCO conductors, because c.c. is essentially characterized as reel-to-reel sequential process for single-crystalline-like triaxially textured films on flexible metal substrate, where it is still the only way to avoid intergrain weaklinks for REBCO wires.
In order to obtain good longitudinal Ic homogeneity of c.c., we had chosen hot-wall type pulsed-laser-deposition (PLD), which realized quite homogeneous crystalline growth conditions for REBCO deposition by furnace-like, nearly equilibrium substrate heating, on sharply textured buffer layers formed by using large-area ion-beam- assisted-deposition (IBAD). As a results, reliable production line of homogeneous non-doped REBCO tapes with lengths over 500 m had been developed with typical Ic performances over 500 A/cm at 77 K in the self-field and over 1000 A/cm (Jc=5-6 MA/cm²) at 30 K in 2 T. Recent years we applied the process for introduction of rod-like APC as BaMO₃ (BMO, M : Zr or Hf) -doped REBCO conductors and found a productive process condition of BMO-doped high-performance tapes with good longitudinal homogeneity. 300-600m long class uniform BaHfO doped EuBCO tapes were formed with productive high growth rate of 20-30 nm/sec, being faster than commercial non-doped conductors, which had also large Ic of 1700-2000A/cm (Jc=7-8 MA/cm²) at 30K, 2T. The angular dependence of in-field Jc properties were investigated in wide temperature range, and strongly c-axis correlated flux pinning were observed over 30K, especially in those films with the growth rate limited lower than 5 nm/sec, where the minimum values of J_c were not so different from high growth rate over 20 nm/sec. These results indicate reliable controllability of deposition parameters on high-rate APC introduction by using hot-wall PLD process. Ic homogeneity analysis of long-length BMO-REBCO c.c. and the microstructural analysis for the growth of BMO rod-structure would be discussed correlated with growth speed, BMO composition, etc.
This Paper includes the results supported by the New Energy and Industrial Technology Development Organization (NEDO).

Large-scaled induction furnaces for non-ferrous metal billets operated at commercial frequency have an energy efficiency of only 50~60% due to the considerable loss from the copper coils used to generate the magnetic fields. Efforts to improve their efficiency are hampered by physical limits. A DC induction heating using HTS magnets has been suggested for achieving higher energy efficiency.
A 10kW-class prototype for an HTS DC induction heater was developed in 2013. Changwon National University and TECHSTEEL had completed a project to develop a 300 kW-class superconducting induction heater(SIH) with HTS magnets in 2017. And Supercoil Co., Ltd., which had established on Sep. in 2016 as a professional development company of the SIH and superconducting magnet, initiated the business of the SIH since 2017.
In this paper, we are going to introduce a 300 kW-class SIH and its performance test results. The HTS magnets were fabricated and excited under the conduction cooling condition, successfully. Now, the SIH with the HTS magnets are being fabricated and tested to heat up the metal billets including both the ferrous and non-ferrous metal billets. The MgB2 magnet was developed for the SIH, too. The performance test results will be presented, in detailed. The all performance test results will be applied for the commercial product of SIH.

High current superconducting wires for large scale power applications and magnets has been one of the most challenging achievements during all the HTS era which encompasses many materials science and engineering challenges. Coated conductors of YBa₂Cu₃O₇ (YBCO) have emerged as the most attractive opportunity to reach unique performances while reducing the cost/performance ratio continues to be a key objective at present. Chemical solution deposition (CSD) is a very competitive cost-effective technique which has been used to obtain nanocomposite films and CCs. In the recent years we have been able to demonstrate the unique potentiality of CSD techniques based on Ink Jet Printing deposition to achieve low cost, low anisotropy and high critical current coated conductors. In this presentation, we will report on recent progress on the development of growth process and enhanced vortex pinning of CSD nanocomposite YBCO films, obtained from colloidal solutions where preformed oxide nanoparticles (NPs) stabilized in the YBCO precursor solutions are used. A thorough investigation correlating the pinning landscape with the defect microstructure has been pursuit through detailed angular dependent in-field critical currents and HRTEM/STEM analysis. We will also report how CSD nanocomposites can be obtained through a new approach based on a transient-liquid assisted growth (TLAG) enabling ultrafast growth rates in the range of 50 nm/s.

Nowadays, the application of high temperature superconducting tapes for energy transport and electrical power devices is limited by its production cost. Therefore, it is of the utmost importance to develop a cost-effective synthetic route of high-throughput YBa₂Cu₃O₇ (YBCO) film growth. Chemical solution deposition of metal organic precursors (CSD-MOD) is known to be a good candidate to fulfil this challenge. YBCO growth rates for the traditional route based on fluorinated precursors are limited by the very low HF out-diffusion during BaF₂ decomposition.
On the other hand, Y₂O₃ diffusion through BaO-CuO melts is known to be very high, thus enabling high growth rates. We propose a CSD route using fluorine free (FF) precursors that, upon proper decomposition of BaCO₃, follows the formation of a transient liquid that results from the eutectic reaction between BaO and CuO. The YBCO layers obtained from this Transient Liquid Assisted Growth (TLAG) method achieve epitaxial films with superconducting transition temperatures of 90 K and critical current densities at 77 K of 3 MA/cm².
In this communication, we present two different methods to grow YBCO with the desired c-axis orientation: the temperature-step route and the PO₂-step route. In the first case, very high heating rates (20-60 °C/s) at a constant pressure force the system to reach the correct region of the phase diagram for the epitaxial growth, and where the precursors (BaCO₃) are fully decomposed. By contrast, in the second route, precursor decomposition and YBCO growth are decoupled, by decomposing BaCO₃ at low PO₂ (~10^-6/10^-7 bar) where YBCO growth is prevented. Afterwards, an increase of O₂ partial pressure results in YBCO epitaxial growth. To understand the growth process and reaction paths, advanced high-resolution Scanning Transmission Electron Microscopy (STEM), In-situ x-ray diffraction, and synchrotron radiation have been employed. Growth rates between 25 and 50 nm/s have been achieved.
Finally, we will also show the compatibility of the TLAG process with the growth of nanocomposites using preformed nanoparticles of BaZrO₃.

Acknowledgements

This work was partially funded by MINECO (project MAT2014-51778-C2); Generalitat de Catalunya (contract 2014SGR-00948); Universitat de Girona (contract MPCUdG2016/059); Center of Excellence award Severo Ochoa (SEV-2015-0496) and ERC-2014-ADG-669504-ULTRASUPERTAPE

In this study, a low-fluorine solution route was developed for the fabrication of YBa₂Cu₃O_7-x(YBCO) coated conductors. In this approach, the solution contains 20-30% of the fluorine of the traditional all-trifluoroacetate solution route. This enables high-speed production of much smoother YBCO films. We systematically investigated the influence of the fluorine content on the phase evolution and superconducting properties of the films. We found that a broad range of fluorine contents can be used to prepare thin high-quality YBCO films. In contrast, for thick YBCO films (with thicknesses over 1μm), F/Ba=2 (in the solution) is optimal to obtain high-J_c YBCO films. So, we used a solution with F/Ba=2. We also developed a deep UV light soaking technique to prepare YBCO films. Instead of thermal pyrolysis of the precursor film, we used UV light to decompose the precursor. This resulted in the constituent elements being uniformly distributed in the films, resulting in enhanced J_c. The addition of water vapor (during the UV irradiation) resulted in the formation of radical O^*. This promoted the removal of carbon residue from the films, and further increased the J_c of the resulting YBCO films.

The implementation of YBa₂Cu₃O_7-δ (YBCO) coated conductors in power applications generally has a potential to change the paradigm in large-scale energy applications. Unfortunately, pure YBCO thin films typically exhibit a strong reduction of the critical current density (J_c) with increasing magnetic field strength caused by vortex motion. There are many different synthetic strategies available to produce superconducting YBCO thin films, of which many are not commercially viable. Here, we collaborated with companies in the field, choosing the approach that will be used for production. It is based on chemical solution deposition (CSD) of a precursor solution with several fluorine contents.
Hereby, the incorporation of preformed nanocrystals (PNCs) as artificial pinning centers was introduced as an adequate approach to achieve flux pinning, preventing the drastic decrease of the critical current density J_c at moderate-to-high magnetic fields as well as its angular dependency on the magnetic field. To achieve this, we have produced nanocomposite thin films starting from PNCs in combination with fluorine-based chemical solution deposition. The use of PNCs generally offers a better control of the final microstructural properties of the nanocomposite films when compared to self-assembled nanoparticles formation during YBCO processing.
In this work, small crystalline single (ZrO₂ and HfO₂) and double (SrTiO₃ and BaZrO₃) metal oxide nanocrystals were synthesized with diameters in the range of 4-10 and capped with hydrophobic ligands to ensure colloidal stability in apolar solvents. However, as the YBCO precursor typically provides a more polar environment, e.g., methanol, an important aspect of this research involves ligand exchange and the appropriate stabilization procedure. We are able to stabilize these nanocrystals in different types of fluorine-based YBCO precursor solutions, leading to highly stable nanocomposite precursors with long shelf-lives.
Afterwards, the YBCO-PNC solutions were deposited on single crystal LaAlO₃ (LAO) substrates via ink-jet printing method. The main focus of this research pointed to understanding the factors which control the microstructure development and the physical properties of the nanocomposite thin films. By strict optimization on both the precursor and processing level, we achieved nanocomposite thin films exhibiting J_c of 4-5 MA/cm² at 77 K in self-field as well as a much smoother decay of J_c as a function of magnetic field. This is reflected by a strong pinning force enhancement (up to 17 GN/m³ at 77 K) and a reduced effective anisotropy compared to undoped YBCO films.
This newly developed approach delivers scalable and high quality superconducting films, capable of meeting the strict requirements for the successful implementation and distribution of coated conductors throughout energy market.

We report the development of high performance thick (Gd,Y)Ba₂Cu₃O_x tapes in a single pass using an Advanced Metal Organic Chemical Vapor Deposition (A-MOCVD) system. Record high critical current (I_c) of over 7200 A/cm at 30 K, 3 T (BIIc) has been achieved in heavily doped (15 mol. % Zr addition), 4.8 µm (Gd,Y)Ba₂Cu₃O_x tapes with a record high lift factor of ~11. This corresponds to an extremely high engineering current density (J_e) of over 7000 A/mm².
In addition, I_c of over 2100 A/4 mm at 4.2 K, 14 T (BIIc) has been achieved in heavily doped (15 mol. % Zr addition), 4.3 µm (Gd,Y)Ba₂Cu₃O_x tapes on IBAD- MgO/LMO substrates .It corresponds to a pinning force (F_p ) of 1.7 TN/m³ and an record engineering current density (J_e) of over 5100 A/mm² which is almost double of the highest value ever reported.
We attribute this achievement to excellent temperature control and laminar flow feasible in A-MOCVD, coupled with optimization of BaZrO₃ nanocolumn growth parameters. High density of BaZrO₃ nanocolumns of average size 4-5 nm and an average spacing of 18-19 nm between them contributes to superior flux pinning properties.

This work is supported by the Department of Energy Advanced Manufacturing Office and the Office of High Energy Physics.

Technical conductors are inherently polycrystalline and do not behave as a single crystal in most cases. In MgB₂ wires, the changing crystallographic orientation of the grains leads, together with the upper critical field anisotropy of this material, to a variation of the local properties when a magnetic field is applied. In the cuprate superconductors on the other hand, this effect is negligible compared to a grain-to-grain misalignment, which drastically reduces the critical current. The strategy for overcoming this limitation was the development of highly textured conductors, most successfully by the coated conductor technology. However, granularity effects are still relevant in some architectures. The grain boundary limitation is weaker in the iron-based superconductors, which may enable cheaper production techniques.
Recent scanning Hall probe measurements are reported addressing granularity effects in different superconductors. While granularity effects can be directly visualized for two-dimensional films, average inter- and intra-granular currents can be obtained from bulk samples. Strategies for optimization of granular materials will be discussed.

Physical phenomena at interfaces of complex oxide hetero-structures have stimulated intense research activities due to the occurrence of a broad range of electric and magnetic functionalities that do not pertain to the constituting single phases alone. Interface effects have been proven to be a powerful tool for improving or even inducing novel functionalities [1, 2]. In the case of interface superconductivity, the interatomic structure relaxation and charge transfer play a key role. In this work, we combine atomic-resolved quantitative STEM imaging with analytical STEM-EELS/EDX analysis to investigate interface effects in La₂CuO₄ (LCO)-based hetero-structures, i.e. superlattices of two-dimensionally (2D) Sr-doped LCO and LCO/La_2-xSr_xNiO₄ (LSNO) hetero-structures, both exhibiting T_c up to 40 K, despite its non-superconducting constituents. We lay emphasis on detailed and quantitative STEM analysis to understand the interplay between interface effects (cation and electron hole redistribution, as well as local lattice and oxygen octahedral distortion) and high-temperature interfacial superconductivity.

With atomically resolved EDXS and EELS analyses, we investigate the cation intermixing, i.e. La/Sr and Ni/Cu, at the interface of superlattices. Oxygen K-edge fine structure analysis were used to quantify the electron hole population. The electron hole profile reveals that charge redistribution occurs at the interface. Atomic-resolved high-angle annular dark-field (HAADF) and annular bright-field (ABF) images, which were simultaneously acquired, were used to evaluate the local lattice and copper-apical-oxygen distortions at these interfaces. The interatomic structure analysis shows that the copper-apical-oxygen distance has a remarkable variation near the interface [3]. We observe an anomalous expansion of the apical oxygen–oxygen distance at the downward side of the interface, and a substantial shrinkage of the apical oxygen–oxygen distance on the upward side in the growth direction of the thin film. Finally, the correlation of these results with the observed T_c will be discussed.

References:
[1] A.Gozar et al., Nature 455 (2008), p.782.
[2] F.Baiutti et al., Nat. Commun. 6 (2015), p. 8586.
[3] Y.Wang et al., ACS Appl. Mater. Interfaces. 8 (2016) p. 6763

Controlling the oxygen content (δ) is a key issue in the fabrication of YBa₂Cu₃O_7-δ (YBCO) films since subtle variations in δ lead to large structural distortions and alter the number of available carriers within the superconducting CuO₂ planes, which ultimately affect the superconducting properties [1]. It is commonly accepted that oxygen vacancies are located within the CuO-chains; however, in this talk, we will present results challenging this hypothesis. We have used aberration-corrected scanning transmission electron microscopy (STEM) to show a periodic and uneven distribution of Oxygen vacancies located at the BaO planes, especially in the proximity of Y₂Ba₄Cu_8-γO_16-d (Y124) intergrowths, the most common defect in YBCO. The presence of these Oxygen vacancies triggers structural changes in the superconducting planes reducing the symmetries of the YBa₂Cu₃O_7-x unit cell: the O_apical-Cu spacing increases as well as the O-Cu-O angle of the CuO₂ planes. Our results are supported by first-principles density-functional theory calculations, which show comparable formation energy of oxygen vacancies in Cu-O chains and in Ba-O planes. Additionally, previous results showed that Y248 intergrowths comes along with Cu and Oxygen vacancies buried within the very same planar defect, which in turn induce a dilute ferromagnetism in the superconducting state of the YBCO [2]. We will discuss the implications of all the observed distortions on the YBCO superconducting properties.

[1] Jorgensen J D, Veal B W, Paulikas A P, Nowicki L J, Crabtree G W, Claus H and Kwok W K 1990 Structural properties of oxygen-deficient YBa2Cu3O7- Phys. Rev. B 41 1863–77
[2] Gazquez J, Guzman R, Mishra R, Bartolomé E, Salafranca J, Magén C, Varela M, Coll M, Palau A, Valvidares S M, Gargiani P, Pellegrin E, Herrero-Martin J, Pennycook S J, Pantelides S T, Puig T and Obradors X 2016 Emerging Diluted Ferromagnetism in High- T _c Superconductors Driven by Point Defect Clusters Adv. Sci. 3 1500295

Authors acknowledge the MICIN (NANOSELECT, CSD2007-00041 and MAT2014-51778-C2-1-R), Generalitat de Catalunya (2014SGR 753 and Xarmae), and the EU (EU-FP7 NMP-LA-2012-280432 EUROTAPES project, Cost Action MP1201 and CA16218 NANOCOHYBRI). They also acknowledge MINECO for the Center of Excellence award Severo Ochoa (SEV-2015-0496).

Combining X-ray tomography and in-field scanning Hall probe microscopy (SHPM), we have succeeded in visualizing local distortion of filaments in IMD processed-MgB₂ wires and their influence on critical current distribution in the MgB₂ wires. High resolution X-ray tomography is a powerful tool to investigate filamentary architecture and local distortion such as sausaging effect and exudation of Mg. From the magnetic image of the magnetized MgB₂ wire measured by the SHPM, we could evaluate spatial distribution of local critical current density, J_c, according to Biot-Savart law. The in-field SHPM allows us to evaluate critical currents quantitatively of the MgB₂ wire with magnetic sheath materials by the measurement where the external magnetic field is higher than the saturation field of the magnetic sheath [1]. The advantage of such magnetic measurements is also to evaluate high current region in which transport method is difficult to apply. Combination with the X-ray tomography and the SHPM leads abundant information on current limiting mechanism of the MgB₂ wires by synergistic effect.

[1] K. Higashikawa et al., IEEE Trans. Appl. Supercond. 26 (2016) 6200804.

Acknowledgements: This study was supported by Japan Science and Technology Agency (JST) as ALCA and also by by Japan Society for the Promotion of Science (JSPS) and Chinese Academy of Sciences (CAS) under the Japan – China Research Cooperative Program.

Continuous and eager efforts on the development of HTS materials have led their practical applications in various fields thus far. Very high J_c exceeding 10¹⁰ Am^-2 at 77 K of coated conductors is now regarded as the lowest standard performance and highly reliable critical current properties of BSCCO tapes have been supporting the successful results of many research projects and development of new superconducting products. In addition, RE123 bulk materials are used for several applications, such as strong magnets, current leads, levitation systems and bearings. These suggests that critical current properties of these HTS materials are enough high against extensive practical applications. On the other hand, high production cost of HTS materials mainly due to a large number of fabrication processes to form highly grain aligned microstructure limits widespread of HTS applications. Our recent studies mainly on the effects of post-annealing at relatively high temperatures, 700~900°C, before oxygen annealing on superconducting properties of HTS indicated that precise control of nonstoichiometric cation composition is crucially important process to derive intrinsic potential of HTS. The cation composition is generally controlled by changing the starting composition, however, it unintentionally varies in the resulting materials depending on the sintering conditions, temperature, partial pressure of oxygen and external pressure. Even for Y123, which has been believed to have an exactly integral cation ratio Y:Ba:Cu=1:2:3, Y ion can substitute for Ba-site up to 2%. In the case of BSCCO, Sr-site is usually substituted by Ca and/or Bi, however, the substitution level can be controlled by applying the post-annealing process. Dramatically positive effects of precise control of cation composition near the integral ratio on superconducting properties, particularly on critical current properties, are shown for RE123, RE247 and BSCCO materials including challenges to reduce the electromagnetic anisotropy by doping and control of dopant level. Furthermore, new strategies to overcome weak-link problem at grain boundaries of HTS and the ultimate potential of HTS materials will be discussed.

We describe an ab initio method to assist prediction of superconductivity in a solid using correlated real and reciprocal space determinants for known or unknown structures. Density functional theory (DFT) calculations for AlB₂-type structures, show that the thermal energy (T_e) associated with phonon anomalies displays one-to-one correspondence to the experimentally determined superconducting transition temperature (T_c)¹. This correspondence is accurate for a range of conditions^1-3 including different isotopes, metal substitution (e.g. Al, Ti or Sc) and changes in pressure (-5 GPa to 20 GPa) for MgB₂. Similarly, we demonstrate correspondence of T_e and T_c for other AlB₂-type structures¹ including BaSi₂ and (Ca_0.5Al_0.5)Si₂.
Using DFT, we evaluate electron density differences invoked by atom displacements aligned with phonon modes in MgB₂ to show that the Fermi energy for the E_2g mode alone accounts for the highly covalent B–B bond charge density distribution. Importantly, the band structure becomes tangential to the Fermi level and the Fermi surface undergoes a topological transition at a critical relative displacement of ~ 0.6% from the equilibrium real space position of boron atoms. The difference in Fermi energies at this critical displacement and at the equilibrium position corresponds to the superconducting energy gap. The volume between tubular σ surfaces in reciprocal space controls the depth of the phonon anomaly and is key to understanding superconductivity because it is directly related to the interplay between light and heavy mass σ bands at G near the Fermi level.
We extend this ab initio method to other superconducting structures including H₃S and the hexaborides covering the range of experimental T_c values from <10 K to ~200 K for multi-element compounds. Hexaboride structures accommodate a wide range of metals with a corresponding range of electronic properties⁴. As with MgB₂ and H₃S, the calculated phonon dispersion (PD) for YB₆ shows a similar proportionate inflexion in the T_2g phonon mode for which the T_e corresponds with measured T_c. These analyses are consistent with “extra” Raman and infra-red (IR) peaks observed in borides^5,6 that imply lower symmetry superlattices are indicators of superconductivity even in cubic structures. Correlation of Fermi surface transitions with key modes in calculated PDs for variations in temperature, pressure or metal substitution shows that electron density distributions along bonds oriented with phonon mode directions are potential determinants of superconductivity.
1. Alarco, J. A., et al., Phys. Chem. Chem. Phys. (2015) 17 (38), 25090.
2. Mackinnon, I. D. R., et al., Comp. Mater. Sci. (2017) 130, 191.
3. Alarco, J. A., et al., Physica C: Supercond. and Applications (2017) 536, 11.
4. Mackinnon, I. D. R., et al., Mod. & Num. Sim. Mater. Sci. (2013) 3 (4), 158.
5. Alarco, J. A., et al., Phys. Chem. Chem. Phys. (2014) 16, 24443.
6. Bando, H., et al., J. Phys. Soc. Jpn. (2011) 80, SA053.

The origin of the spectacular enhancement of the effective critical current density with increasing external magnetic field H, leading to a second magnetization peak (SMP) on the dc magnetic hysteresis curves of superconducting single crystals with random pinning, is still not clear. Many interesting SMP models and mechanisms have been proposed and considered so far, and it is believed at present that this effect is system dependent. However, by analyzing the dc magnetization relaxation in the SMP domain for various single crystal specimens (superconducting cuprates and iron-based superconductors), of different pinning strength, with H oriented along the crystallographic c axis or perpendicular to it, we found several sample independent, characteristic aspects, such as the absence of single-vortex collective pinning around the SMP onset field, and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior and the peculiar disappearance of the SMP in certain underdoped cuprates and pnictides (attributed by us to the presence of SDW stripe-like static order) support strongly a continuous pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

The parent compound of high-T_c superconducting cuprates is a unique Mott state consisting of layers of spin-1 / 2 ions arranged on a square lattice, with a record high antiferromagnetic coupling within the layers, strong covalency between transition metal (TM) and ligands and no orbital degeneracy. There have been several attempts to replicate these characteristics with different TM ions. One proposal has been to use nickel(I) in place of copper(II). LaNiO₂ is isoelectronic and isostructural with infinite layer parent cuprates, however, it lacks the strong covalent character between TM and ligand and the antiferromagnetic order has not been found. Sr₂IrO₄ has many similarities with cuprates including a robust antiferromagnetic order of spin-1/2 pseudospins. However, the correlated insulator character is much weaker and spin-orbit coupling effects dominate. I will discuss an alternative route where, to a large extent, these problems are solved and the physics of Cu(II) ions emerges without Cu.

The absorption of hydrogen and subsequent precipitation of hydrides degrades the superconductive properties of niobium. Although addition of dopant elements, particularly nitrogen, have shown improvement in the quality factor of superconducting radio frequency (SRF) niobium cavities, the underlying mechanisms associated with kinetics of hydrogen and the thermodynamic stability of hydride precipitates are not well known. Here, through first principles calculations, we report the effects of nitrogen doping on the energetic preference for hydrogen to occupy interstitial sites and the stability of niobium hydride. In particular, we show that the presence of nearby nitrogen significantly increased the energy barrier for hydrogen diffusion from one particular tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable up on addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, we find that nitrogen has a tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to an unstable state for both hydrogen and hydride. This explains the experimental observation i.e., niobium hydride is less stable in nitrogen doped niobium. Therefore, the presence of nitrogen during the processing of SRF cavities can play a critical role in controlling hydride precipitation and subsequent SRF properties.

For practical use it is essential to make Fe-based superconducting materials into wire and tape conductors with sufficient current carrying capability, which is limited by misaligned grains inside the conductors. The development of Fe-based superconducting wires at IEECAS has progressed rapidly resulting in improving the wire performance. A record transport critical current density (J_c) reaching 1.5 ×10⁵ A/cm² (I_c = 437 A) at 4.2 K and 10 T in Ba_0.6K_0.4Fe₂As₂ (Ba-122) tapes by texturing the grain orientation with optimized hot-press technique. The transport J_c measured at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5 ×10⁴ A/cm². Moreover, at 20 K and 5 T the transport J_c is also as high as 5.4 ×10⁴ A/cm², showing a promising application potential in moderate temperature range which can be reached by liquid hydrogen or cryogenic cooling. The high-performance PIT Ba-122 tapes in this work suggest iron-based superconductors to be a strong potential conductor for high-field applications.

Significant progress has been made in fabricating high-quality epitaxial thin films of iron-based superconductors. Strain engineering offers the possibility of tailoring the structural distortions at the atomic scale and enhancing superconducting properties. Here, we report that tetrahedral geometry driven by thin film strain leads to a significant enhancement of the superconducting transition temperature (T_c) of optimal Co-doped epitaxial BaFe₂As₂ thin films above the value of the bulk single crystals. We have found that the As-Fe-As bond angles were strongly modified by both epitaxial and thermal strains caused by the temperature-dependent lattice mismatch between BaFe₂As₂ thin films and the substrates. Synchrotron x-ray diffraction and resonant scattering demonstrate that the As-Fe-As bond angle and T_c are systematically tuned by in-plain strain and reach maximum T_c at the optimum bond angle of 109.5^o. Strain engineering can provide a path toward tailoring superconducting properties and understanding superconductivity in other Fe-based superconducting thin films such as monolayer FeSe.

This work has been done in collaboration with J. H. Kang, P. J. Ryan, J. W. Lee, J. W. Kim, Y. Choi, J. Jiang, E. E. Hellstrom, D. C. Larbalestier.

** This was supported by the DOE Office of Basic Energy Sciences under award number DE-FG02-06ER46327.

Superconductivity has been reversibly induced in undoped CaFe₂As₂ (Ca122) single crystals with T_c at ~25 K at ambient pressure and up to 30 K at 1.7 GPa. We found that Ca122 can be stabilized in two distinct tetragonal (T) phases: PI with a nonmagnetic collapsed tetragonal (cT) phase at low temperature and PII with an antiferromagnetic orthorhombic (O) phase at low temperature. Neither phase at ambient pressure is superconducting down to 2 K. However, systematic annealing for different time periods at 350 °C on the as-synthesized crystals reveals the emergence of superconductivity over a narrow time window. Detailed X-ray diffraction profile analyses further reveal mesoscopically stacked layers of the PI and the PII phases. The deduced interface density correlates well with the superconducting volume measured. The transport anomalies of the T-cT transition and the T-O transition are gradually suppressed over the superconductive region, presumably due to the interface interactions between the nonmagnetic metallic cT phase and the antiferromagnetic O phase. Our most recent STEM data display 8-20 nm domains in the superconducting Ca122 samples at 90 K. The results provide the most direct evidence to date for interface-enhanced superconductivity in undoped Ca122, consistent with the recent theoretical prediction. References: K. Zhao et al., PNAS 113, 12968 (2016); S. Y. Huyan et al., in preparation.

NdFeAs(O,F) thin films of around 50 nm thickness were grown by molecular beam epitaxy on MgO single crystal. The F doping was achieved by F-diffusion from a NdOF over-layer into the undoped NdFeAsO layer. The film showed a sharp cube-on-cube texture. Electrical transport properties were measured on laser-cut micro-bridges in 4-point geometry and maximum Lorentz force configuration in magnetic fields up to 14 T.
The T-dependent E(J) curves at constant magnetic field can by scaled within the framework of the vortex glass-liquid transition. Best scaling is achieved for 3D and critical exponent ν ~ 1.3 and z ~ 9. Whereas ν is in the expected range of 1.0-2.0, z is slightly above the expected range of 4.0-6.0. The reason for this increased value might lie in the distribution of pinning energies and, hence, local J_c variation. The activated flux motion within the vortex liquid range is characterized by analyzing the scaling behavior of the temperature-dependent resistivity curves as well as their derivatives.
The work was partly supported by the JSPS Grant-in-Aid for Scientific Research (B) Grant Number 16H04646.

It was recently demonstrated that monolayer FeSe on a SrTiO₃ substrate is a superconductor with T_c between 60 and 100 K, compared to 8 K in bulk FeSe. This is in contrast to the expected behavior; thinning a superconductor typically reduces T_c. Similar results have been obtained in monolayer FeSe deposited on BaTiO₃ and anatase TiO₂. In contrast, T_c has been measured to be only 3.7 K in bi-layer FeSe deposited on graphene (extrapolated to about 2 K in a monolayer), pointing to the major role of the interface in enhancing superconductivity. Here we determine the atomic structure of an interfacial layer and identify its role in driving the increase in T_c using a combination of quantum mechanical calculations and scanning transmission electron microscopy. Within our DFT calculations, this interfacial layer hosts long-range magnetic and orbital order not found in the typical TiO₂ surface termination nor in previously identified surface reconstructions. Our calculations suggest the presence of a weak magnetic coupling between the interfacial monolayer and the deposited FeSe film, yielding a rich phase space of possible magnetic states. Interactions between this interfacial monolayer and FeSe generate symmetry-breaking distortions in the film that are favorable for increasing T_c and are not present in other possible FeSe / STO interface structures. We propose that this may provide a path forward toward the design and enhancement of other two-dimensional superconductors.

DFT calculations were supported by U.S. DOE grant DE-FG02-09ER46554 and the McMinn endowment at Vanderbilt University (HS, STP). They were performed at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (HS, STP). Work at ORNL is sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (DNL, AYB, VRC, MFC). Work at West Virginia University (ZG, LL) is supported by the U.S. National Science Foundation, Division of Materials Research (DMR-1335215).

We report high-temperature superconductivity in B-doped amorphous quenched carbon (Q-carbon). This phase is formed after nanosecond laser melting of B-doped amorphous carbon films in a super undercooled state and followed by rapid quenching. Magnetic susceptibility measurements show the characteristics of type-II Bardeen-Cooper-Schrieffer superconductivity with a superconducting transition temperature (Tc) of 36.0±0.5 K for 17.0±1.0 atomic percent boron concentration. This value is significantly higher than the best experimentally reported Tc of 11 K for crystalline B-doped diamond. We argue that the quenching from metallic carbon liquid leads to a stronger electron-phonon coupling due to close packing of carbon atoms with higher density of states at the Fermi level. With these results, we propose that the non-equilibrium undercooling assisted synthesis method can be used to fabricate highly doped materials which provide greatly enhanced superconducting properties.
[1] Bhaumik, A; Sachan, R; Narayan, J. High-Temperature Superconductivity in Boron-Doped Q-Carbon. ACS Nano 2017, DOI: 10.1021/acsnano.7b01294.

Thin films of superconducting magnesium diboride are required for several applications. However, their fabrication can be challenging, particularly on amorphous or polycrystalline substrates when the film growth is inherently non-epitaxial. Here, we describe the fabrication and properties of 50-600-nm-thick MgB2 films on glassy carbon substrates, through optimizing parameters of Mg vapor annealing of sputter-deposited amorphous B films. Our results reveal a critical role of both the initial B film thickness and the temperature-time profile on the microstructure, elemental composition, and superconducting properties of the resultant films. We discuss the challenges related to the chemical reactivity of ultrathin B films, the control of nucleation and growth of MgB2 crystallites, and the formation of boron carbides at the film-substrate interface. We demonstrate the existence of a window of process parameters resulting in smooth superconducting films with thicknesses down to 50 nm. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

REBCO superconducting tapes with different oxide nanoparticle additions have been fabricated on IBAD/MgO tapes by pulsed laser deposition process (PLD). For stoichiometric (123) REBCO films, it was found that when REBCO film thickness was smaller than 1.0 micrometer, REBCO superconducting films had a high critical current density J_c of 5.0x10⁶ A/cm² (at 77 K, self magnetic field) . As REBCO film thickness increased over 1.0 micrometer, the J_c values of REBCO films decreased with increasing of REBCO film thickness. X-ray diffraction (XRD) and high resolution scanning electron microscope (SEM) observation showed that a-axis orientation was easily formed in thick REBCO films. At higher substrate temperatures, BaCuO3 particles were also found in REBCO films. The main reason for lower J_c of REBCO thick films was broad grain boundaries. SEM and AFM observations showed that thick REBCO films had micrometer-scale grains and relatively broad grain boundaries which blocked superconducting current paths. In order to control the grain size and grain boundary width of REBCO thick films as well as to improve in-field superconducting properties, Y₂O₃, BaHfO₃ and BaZrO₃ addition effects were investigated. AFM, SEM and TEM images showed the presence of nano-scale Y₂O₃, BaHfO₃, and BaZrO₃ oxide particles in REBCO superconducting film. During deposition, these nanoparticles on REBCO film surface changed REBCO epitaxial growth process and acted as new crystal nucleus. By adding oxide nanoparticles into REBCO films, not only in-field superconducting properties but also surface morphology of REBCO films were obviously improved, which makes the growth of REBCO thick films to be easier. Four probe I-V curve measurements showed REBCO tapes with artificial pinning centers had excellent in-field performance. For 1.0 cm wide tapes, I_c was over 1000A (4.2K and 12T, H//c).

REBa₂Cu₃O_y coated conductors have been widely manufactured by various methods. Among them, fluorine-free MOD (FF-MOD) method is one of the most cost-effective processes to prepare REBCO tapes because of simple chemical reaction under ambient pressure with very high processing rate. Our recent studies revealed that addition of moderate amount of hydrochloric acid to the starting solution resulted in generation of bi-axially aligned oxychloride Ba₂Cu₃O₄Cl₂ (Ba2342) fine crystals which assist biaxial growth of YBCO layers^[1][2]. Therefore, highly textured YBCO films can be successfully prepared with high reproducibility in Cl-added FF-MOD method. In this study, further improvement of critical current(I_c) characteristics has been attempted for FF-MOD processed YBCO thin films on clad substrates covered by biaxially oriented oxide layers by increasing the thickness of YBCO layer.
Propionate-based solution with a nominal composition of Y: Ba: Cu: Cl = 1: 2.1: 3.15: 0.1 was coated on clad substrates by spin-coating. Coating and calcination at 500°C under the flowing oxygen atmosphere were repeated 1–9 times to control the film thickness. The calcined films were sintered in the tube furnace under flowing O₂/Ar gas with P_O2 = 10 Pa, followed by oxygen annealing at 450°C for 12 h. I_c values (A/cm) of the prepared films were evaluated by an inductive method in liquid nitrogen without an external field.
Optimizations of heat-treatment processes, such as calcination, sintering and oxygen annealing, for YBCO films (~0.5 mm^t) resulted in a decrease in sum of all the heat-treatment time to half of the typical one with maintaining relatively high I_c ~90 A/cm. I_c of YBCO films with various thickness (0.17–1.5 mm^t) sintered at 800°C for 1 h was highest (~100 A/cm) for a film with ~0.8 mm^t and thicker films more than 1.0 mm^t showed relatively low I_c due to an increase of impurity particles and/or generation of cracks. On the other hand, intermediate sintering was found to be effective for suppressing degradation of I_c for thick YBCO films. A YBCO film (~1.0 mm^t) sintered twice exhibited higher I_c ~126 A/cm.

[1] T. Motoki et al., Supercond. Sci. Technol. 27 095017 (2014).
[2] T. Motoki et al., Supercond. Sci. Technol. 29 015006 (2016).

YBa₂Cu₃O_7-δ (YBCO) coated conductors for electric power applications are demanding highly flexible and low-cost manufacturing processes. Up to now, textured superconducting YBCO films with the desired properties for the coated conductor architecture are mostly realized via vacuum processes such as pulsed laser deposition (PLD) and metal-organic chemical vapor deposition. These require costly high-vacuum systems, which are not attractive for industrial scale. Chemical solution deposition (CSD) is a non-vacuum technique and can fulfill demands such as cost-effectiveness, e.g. through high yield, and easy scalability. However, the pathway to fabricate YBCO nanocomposite films with relevant pinning properties does not seem to be easy because of the necessity to solve several critical issues related to YBCO growth. In the case of the self-assembled nanocomposite films produced via the PLD method, it has been already shown that the precise size tuning of secondary phases in YBCO matrix leads to improved pinning properties.
In this work, two different methods to synthesize HfO₂ nanocrystals were introduced to reveal the influence of nanocrystal morphology and size on the defect state of YBCO matrix. This approach comes with some new specific challenges to tune the particle size for the CSD-based YBCO nanocomposite film starting from preformed nanocrystals. The solvothermal heating-up synthesis yielded highly crystalline nanorod-like HfO₂ nanocrystals with a size of 2.6 nm in diameter and 8.0 nm in length. The microwave-assisted synthesis delivered highly crystalline HfO₂ nanocrystals but now are spherical with a size of 6-8 nm in diameter.
For these two types of monoclinic HfO₂ nanocrystals, we have shown the possibility to deposit the YBCO nanocomposite thin films starting from preformed HfO₂ nanocrystals as nano-sized defects for flux pinning in combination with environmentally benign low-fluorine YBCO precursor solution on single crystal LaAlO₃ substrates using a spin-coating technique. This CSD approach has been shown that the nanocrystal size and its distribution are the key parameters for a better control of structural defects in the YBCO matrix. Small particles of 5-20 nm would tend to promote formation of small planar defects (stacking faults) surrounding the particles, yielding an increase of microstrain and leading to high pinning force density of up to 17 GN m^-3. Large particles (>25 nm) in the YBCO matrix could lead to formation of larger stacking faults leading to degradation of the critical current density in the YBCO nanocomposite films.

In the previous investigation, the development of REBa₂Cu₃O_y coated conductor (REBCO-CC) was promoted by using multi insulative buffer layer to restrain metallic element diffusion from the metallic tape substrate and to improve crystal orientation. Consequently, depositing of high-cost Ag layer to stabilize and protect from damage due to quenches are needed, and then it is difficult for production of REBCO-CC to lowering of overall cost. Recently, Doi et al suggested the novel REBCO-CC without Ag stabilizing layer structure, which substituted conductive buffer layer such as Nb:SrTiO₃ and Nb:TiO₂ for insulative buffer on Ni-electroplated Cube-textured Cu tape.
On the other, we suggested the production method of REBCO film via low-temperature molten alkaline hydroxide, is called “KOH flux method”. The KOH flux method can grow the biaxially crystal-aligned REBCO film on single crystalline substrate at below 500°C. Therefore, it is prospective technique for production of REBCO-CC to cost lowering. Moreover, recently we succeeded in the low-temperature fabrication of LRE–Ni–O (LRE = La, Nd) compounds by KOH flux method. The LRE–M–O compound has a conductive property in particularly case, accordingly, applying LRE–M–O layer to conductive buffer layer is expected. Conclusively, we can suggest that the novel technique for production of whole REBCO-CC by using low-temperature KOH flux method to fabricate conductive buffer layer and REBCO layer are established. In this report, we discussed correlation between depositional condition and obtaining phase, orientation for LRE–M–O films.
For preparing starting materials, LRE₂O₃ (LRE = La, Nd) and M–O powders were weighed with molar ratios of LRE : M = 1 : 1 and 2 : 1 respectively, and calcined at 1000°C for 24 h. Starting materials, KOH of 400 wt% to starting materials and LaAlO₃ single crystalline substrate were put into alumina crucible, and heat-treated at 400–700°C for 12 h. All steps were performed at ambient pressure. After cooling to room temperature, the film and powder samples were extracted from the flux, and the obtained samples were washed by ultrasonic cleaning with distilled water and ethanol to eliminate the KOH and K₂CO₃. The grown phase, orientation, and lattice parameters were determined by X-ray diffraction (XRD) with a CuKa source. Surface morphology was obtained by optical microscope and scanning electron microscope (SEM).
In the case of La–Ni–O films, the LaNiO₃ phase was synthesized at various depositional conditions, although phase stability and orientation deteriorated with increasing fabrication temperature. On the others, Nd–Ni–O films showed that the Nd₂NiO₄ phase was obtained mainly above 500°C in both Nd : Ni mixture ratios

The intense research activity underway in iron-based superconductors, has included significant attention to FeSe due in part to its relative structural simplicity and promising characteristics for elucidating the physics of superconductivity in these materials. These characteristics include the highest critical temperature observed so far in iron-based superconductors (T_c ~ 65 K for an FeSe monolayer grown on SrTiO₃), a nematic phase that is not suppressed when frustrated magnetic interactions suppress magnetic ordering, and Cooper pairing with orbital selectivity. FeSe-based alloys and chemical doping of FeSe can lead to additional attractive features such as control of T_c, altered nematic phase structure, possibly distinct pairing of charge carriers, and high H_c and J_c values, which are valuable in high magnetic field applications. In particular, doping of FeSe with Cu has been shown to be an effective way of switching superconductivity on and off in bulk samples of Fe_1-xCu_xSe. In this work, Fe_1-xCu_xSe thin films were grown on MgO (100) substrates via pulsed laser deposition. A KrF excimer laser (1.4–3.4 J/cm²) was used to ablate a pressed, sintered target produced by mixing powders of FeSe and Cu₂Se precursor compounds at various concentrations. Targets were sintered in sealed quartz ampules at 700°C for 12 hours. Targets were ablated at pressures below 3.0x10^-6 Torr with substrate temperature kept at 500°C. The thickness of all films was targeted at 400 nm as determined by deposition rate and scanning electron microscopy thickness calibrations. Films are analyzed by atomic force microscopy for surface morphology, and X-ray diffraction to characterize the crystalline structure of the films. We show that the surface morphology and crystallographic phase and orientation of the Fe_1-xCu_xSe films vary greatly with deposition conditions. Virtually all samples grown with x = 0 exhibit both, the tetragonal β-FeSe phase and the hexagonal δ-FeSe phase. It is also found that increasing the laser fluence in the 1.4–3.4 J/cm² range, with the substrate temperature kept at 500°C, increased the fraction of β-FeSe in the films. Features of Stranski–Krastanov island growth are apparent in these samples for all deposition conditions explored with x = 0, suggesting that c-axis oriented β-FeSe islands grow on a δ-FeSe facet with a close epitaxial relationship to the tetragonal phase. This effect may allow the controllable growth of arrays of superconducting dots. Fe_1-xCu_xSe films with x = 0.010 and x = 0.015 are shown to be exclusively tetragonal, with the c-axis of the tetragonal structure oriented normal to the MgO substrate. These synthesis conditions correspond to the enrichment of selenium in the ablation target, since the Cu precursor used in the process (Cu₂Se powder) is also a source of Se, which is consistent with the known sensitivity of the Fe:Se ratio on the stability of the crystal structure of this FeSe-based system.

Ag-sheathed Bi2223 long tapes have been extensively used for various applications thus far particularly after the development of DI-BSCCO in 2004. DI-BSCCO is fabricated by two step sintering processes, 1st sintering under ambient pressure (P_O2 ~ 8 kPa, ~840°C, ~100 h) to form the Bi2223 phase and 2nd sintering under high gas pressure (P_total = 30 MPa, P_O2 ~ 8 kPa, ~840°C, ~100 h) after flat rolling for densification and controlling grain orientation. The overpressure sintering suppresses both generation of voids and degradation of c-axis grain orientation due to grain growth of Bi2223 [1]. Typical I_c of DI-BSCCO is 200 A at 77 K in self field [2]. On the other hand, sintering under low P_O2 (< 5 kPa) was found to promote generation of Bi2223 from the precursor in our previous study[3]. Therefore, high I_c tape fabrication process could be designed by only 1st sintering under over pressure and moderately low P_O2 (< 5 kPa). Based on these backgrounds, effect of over pressure in 1st sintering process on phase formation and superconducting properties of Bi2223 tapes have been investigated to develop high I_c Bi2223 tapes in a short fabrication time.
Through the systematic studies on 1st sintering as functions of temperature, time, P_total and P_O2, the phase formation rate of Bi2223 phase was found to monotonically decrease with an increase of P_total. An increase in tapes thickness after sintering is smaller for tapes sintered under over pressure than that of tapes sintered under ambient pressure. These indicate that phase formation of Bi2223 somehow suppressed in a highly dense oxide filament, while the densification of filaments is preferable to achieve high I_c. In addition, c-axis lengths of tapes sintered under over pressure are apparently shorter than the standard tapes, suggesting promotion of substitutions of (Ca, Bi) and Pb for the Sr site and Bi site, respectively. Critical current properties of the tapes will be reported including the effect of post-annealing to control cation composition as well as carrier doping state.

[1] K. Sato, S. Kobayashi and T. Nakashima, Jpn. J. Appl. Phys. 51 (2012) 010006.
[2] T. Nakashima, S. Kobayashi, T. Kagiyama, M. Kikuchi, S. Yamade, K. Hayashi, K. Sato, G. Osabe and J. Fujikami, Cryogenics 52 (2012) 713–718.
[3] R.Tajima, J. Shimoyama, A. Yamamoto, H. Ogino, K. Kishio, T. Nakashima, S. Kobayashi and K. Hayashi, IEEE Trans. Appl. Supercond. 23 (2013).

Nowadays 1D semiconducting nanowire (NW) networks attracted significant interest because of their potential applications in the single-photon detectors, quantum-computing circuits and SQUID-like sensors. There are several approaches to fabricate 1D semiconducting NW network exist. In bottom-up approach ultra thin film of superconducting material deposited on top of the porous template with nanometer sized cells – porous alumina or porous silicon (PS), for example – and due to the very small thickness deposited material cover only surface of the template forming ordered network of superconducting NWs. However in another approach arrays of ferromagnetic NWs could be introduced in ordered porous templates providing regular magnetic arrays with characteristic dimensions, i.e. diameter of the NWs and nearest neighbor distance, in the nanometer range. Ordered array of ferromagnetic NWs serves as a source of periodic magnetic pinning centers in the superconducting film and matching effects between the artificial non-superconducting regular structure and the vortex lattice can be observed. The system behaves like a superconducting NW network when the coherence length is of the same order as the width of the channels between the non-superconducting areas. In this work we investigate the superconducting properties of a 30 nm thick Nb film sputtered on composite material consisting of an array of Ni NWs grown into a PS template. Nb film was electrically isolated from the ferromagnetic composite by an 8 nm insulating alumina layer.
Study of T_c(H) phase diagrams constructed by measuring resistive transitions of obtained samples in various perpendicular magnetic fields for several resistive transition criteria of the normal state resistance (10 – 90 % of normal resistance) revealed pronounced anomaly in the magnetic field values range 0.19-0.23 T where the critical temperature apparently increases with respect to what is conventionally expected, reaching a value that is almost equal to zero field critical temperature. Similar anomaly with lower amplitude is also present at higher fields, in the range 0.53-0.64 T. Samples were Ni NWs electrically interconnected with each other do not show such behavior. Measurements of magnetoresistance and voltammetric characteristics revealed that observed matching effects are more likely caused by the Little-Parks effect more than by the vortex pinning. Therefore, the system behaves like a 1D superconducting NW network where the confinement of the Cooper pairs is not caused by geometrical constrictions, but rather by the stray fields of the magnetic dipoles that modify the topology of the plain film creating regions where the superconductivity is strongly depressed.
This work is supported by Belarusian Republican Foundation for Fundamental Research, grant T16M-012.

Free-standing 2D crystals mechanically cleaved from van der Waals solids exhibit unprecedented high crystal quality and macroscopic continuity which not only renovate the cognition that 2D long-range crystalline order cannot exist at finite temperature but also reveal the dramatic effect of dimensional crossovers on intrinsic physical properties. However, current progress is primarily limited to those typical strongly-anisotropic layered lattices and for the predominated 3D non-layered lattices whether their isolated atomic planes could survive in principle and if so how to two-dimensionalize those isotropic solids from their high symmetry parent crystals remains fundamental question and significant challenge. Herein, we theoretically proposed the quantum electronic stability of freestanding rock-salt ZrN nanosheet with critical-thickness of three atomic bilayers based on the electronic growth model. Along the way, we further experimentally realized ZrN vdW-like crystal via a bilayer-by-bilayer growth mode under CaH₂-assisted topotactic reaction. The ZrN vdW-like crystals can be mechanically exfoliated and transferred in the same way as natural layered vdW materials and the step height in exfoliated layers is in good agreement with the height of critical-thickness calculated. The exfoliated high quality ZrN nanosheets with a few nanometres thick exhibits dimensional crossover effect of emerging spin density wave and 2D superconductivity with the unconventional upper critical field beyond the Pauli paramagnetic limit, which suggests a role for spin fluctuations in the pairing mechanism in dimensionally confined superconductors. The existence of critical thickness stability for 2D freestanding lattice and mild topotactic transformation process can be the general strategy to design and fabricate other high-quality 2D crystals in isotropic lattice, which further enrich the arena of artificial 2D materials and their confined new physics.

Because the topological protection of conducting surface states stems from the crystal symmetry for topological crystalline insulators (TCIs), crystal defects on the surface of TCI SnTe can strongly influence its topological and transport properties. As a derivative, In-doped SnTe was recently rediscovered as a candidate for time-reversal-invariant topological superconductors (TSCs) [1]. However, the high concentration of Sn vacancies inhibits surface states to be easily revealed [2], and the inhomogeneous distribution of In dopants in conjunction with catalyst impurities induced by chemical vapor deposition (CVD) may result in mixed behaviors of superconductivity [3]. Therefore, to manipulate topological and superconducting properties of In-doped SnTe, it is important to investigate how point defects, dislocations, surface topography and transport properties correlate with each other. We propose that the crystalline quality can be improved by suppressing the formation of surface defects on SnTe and In-doped SnTe micro/nanocrystals synthesized by CVD [4].

Our work focuses on the study of morphology-dependent superconductivity of In-doped SnTe nanostructures [3]. Superconducting transitions are obtained in transport measurements of In-doped SnTe nanostructures synthesized by CVD using gold nanoparticles as a catalyst. For nanoplates, complete superconducting transitions are observed at ~2.0 K, whereas lower-dimensional nanoribbons and nanowires present gradual and possible multiple transitions at lower temperatures of ~1.4-1.75 K. Saturating resistances in some nanowires also indicate incomplete superconducting transitions. We propose that inhomogeneity, such as surface In-doping inhomogeneity and gold impurities, is exaggerated in the transport measurements of smaller nanostructures, leading to variations of superconducting behaviors between different morphologies.

Moreover, to investigate surface defects that may degrade the topological characteristics of In-doped SnTe, we characterize surface pits, open cores and steps on {100} and {111} surfaces of SnTe micro/nanocrystals with and without In doping [4]. We show that enlarging, deepening, and faceting of the surface pits are enabled by sublimation of open cores and anisotropic movement of steps during natural cooling after the CVD growth. Distinct morphologies of surface pits arise from the crystal symmetry, surface termination, presence of In dopants, and dislocations of SnTe. Fast cooling suppresses the formation of surface pits, although growth conditions need to be optimized to decrease the density of dislocations that are responsible for the surface defects of SnTe micro/nanocrystals.

References:

[1] Sasaki,S., and Yoichi Ando. Crystal Growth & Design 15 (2015): 2748-2752.
[2] Novak, M., et al.. Physical Review B 88 (2013) 140502.
[3] Kumaravadivel P., et al.. APL Materials 5 (2017) 076110.
[4] Liu, P. et al.. Journal of Physics and Chemistry of Solids, DOI: 10.1016/j.jpcs.2017.12.016 (2017).

It is shown that many transition metal carbides (TMCs), such as Mo2C, WC, TaC and NbC, exhibit superconductivity which strongly depends on the crystal thickness. Here we show the CVD synthesis of thin Mo2C crystals on Cu foil by annealing Mo-Cu substrates in CVD system under methane flow. Grown crystals are characterized using Optical Microscope, Raman Spectroscopy, AFM, SEM with EDS and XRD. The effects of processing parameters on the thickness and morphology of the Mo2C crystals are discussed in order to explain the mechanism. This techniques may open the door to synthesis of other 2D TMC crystals.

We have studied from the viewpoints of high speed deposition, large area, high performance of REBa₂Cu₃O_y coated conductors. High performance is determinedabout by optimum shapes of the artificial pinning centers (APC) in REBa₂Cu₃O_y (REBCO) coated conductors towards superconducting magnets operating at temperatures of 77 K or less.and lower temperature. Superconducting properties have been changedvary depending on the by different kind and addition amountquantity of BaMO₃ added to REBCO. Therefore, we study the changes in theof shapes of nanorods shapedue to the difference in theof nature of additives and growth temperature. In addition, weand aim to control the APC having anwith the optimum shape that matchesing the operating temperature.
The high flux pinning performance was obtained for a 3.8vol.% BaHfO₃ (BHO)-doped SmBa₂Cu₃O_y (SmBCO) on IBAD-MgO. At 77.3 K, the irreversibility field (B_irr) of 16.8 T and the maximum flux pinning force density (F_p) of 32.5 GN/m³ (B//c) were achieved. In addition, the maximum F_p values of 400 GN/m³ and 120 GN/m³ for B//c were realized at 40 K and 65 K.
In particular, we describe the shape control of nanorods in SmBCO coated conductors by employingusing low temperature growth (LTG) technology using seed layers. From the cross- sectional TEM observations, weit was confirmed that using the LTG technique, the BHO nanorods, which were comparatively thin in diameter and short in length, formed athe fireworks structure in the case of SmBCO coated conductors. The superconducting properties in the magnetic field of the SmBCO- coated conductor on IBAD-MgO with the optimum amount of BHO showed that F_p^max = 1.5 TN / m ³ at 4.2K.
The high-speed growth technique for high performance REBCO coated conductor of the next task is an important subject. In this presentation, we will describe the development of the coated conductor fabricated with a combination a high repetition rate and a novel PLD growth. Thus far we have proposed a vapor-liquid-solid (VLS) method in which a thin liquid layer is interposed in the growing REBCO films with high quality and high speed. A VLS growth technique enables REBCO coated conductor to grow rapidly compared with the conventional PLD method. Furthermore, we will present the results such as introduction of APC for improving not only the fabrication speed but also the performance in the magnetic field of the VLS-RE123 coated conductor on the IBAD tape.

This work was partly supported by a Grant-in-Aid for Scientific Research (15H04252 and 16H04512). A part of this work includes the results supported by the ALCA project of the Japan Science and Technology Agency (JST) and NU-AIST alliance project.

The addition of nanosize defects to YBa₂Cu₃O_7-z (Y-Ba-Cu-O or YBCO) superconductor thin films have been studied by many groups world-wide, to enhance flux pinning and strongly increase critical current densities (J_cs). Since the first publications of this field in 2003 and 2004, over 6,200 citations have been listed for the subfield ‘flux pinning YBCO’, and the subfield ‘flux pinning’ has over 60,000 citations (source, Google Scholar). Despite many excellent systems studied, there are still new types of defect additions, flux pinning mechanisms, and processing methodologies to explore. This paper reviews and summaries studies in our lab from 2003 to 2018 on multiple YBCO+M+N systems, and presents recent interesting results on YBCO+Y₂BaCuO₅ films, achieving to our knowledge the largest defect sizes > 40 nm in YBCO-films so far. The pinning of this system is surprisingly different, by attaining > 10 vol% additions and strongly enhancing both strong anisotropic and weak isotropic pinning simultaneously. Thin films were made by pulsed laser deposition of (M/YBCO)_X multilayer and (YBCO)_1-x(M,N)_x single-target films. Results and comparisons of flux pinning for different realms of T = 4-77 K, H = 0-9 T, and Theta = 0-90 degrees will be presented, and correlated with microstructure studies.

This work was supported by the AFRL Aerospace Systems Directorate and the Air Force Office Scientific Research (AFOSR).

Cuprate (RE)Ba₂Cu₃O_7-d (RE = rare earth elements) high temperature superconducting (HTS) cables offer powerful opportunities for increasing capacity, reliability, and efficiency of the electricity grid. HTS coils can provide an alternative to rare-earth permanent magnets used in rotary machines and generators. Here, we present our recent studies carried out at Brookhaven National Laboratory’s Tandem Van de Graaff facility. We demonstrated a roll-to-roll irradiation process on production-scale cuprate HTS coated conductors that resulted in uniform enhancement of critical current up to 77 K at the dosage and ion energies readily accessible with commercial electrostatic generators. While great progress has been made in the applications of cuprate HTS, iron-based superconductors have attracted a great deal of interests in both fundamental physics and potential applications. We have grown iron-chalcogenide superconducting films on various single crystal substrates and metal substrates with enhanced transition temperature T_c, and carry high critical current density J_c.^1,2 Recently, we developed a new route for simultaneous increase of T_c and J_c in iron-based HTS films by low-energy proton irradiation. Extensive transmission electron microscopy analysis provides direct atomic-scale imaging of cascade defects and the surrounding nanoscale strain field produced by low-energy proton irradiation.³ T_c is enhanced due to the nanoscale compressive strain induced by the irradiations and proximity effect, whereas J_c is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of J_c enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface. This robust, scalable and practical route opens up the possibility to improve both T_c and J_c in all superconductors [Ref. 1) Rep. Prog. Phys. 74, 124510 (2011). 2) Nat. Commun. 4, 1347 (2013). 3) Nat. Commun. 7, 13036 (2016).]

The author acknowledges the funding support from US DOE Office of Science, Materials Sciences and Engineering Division, and the US DOE, Advanced Research Project Agency-Energy, and collaborations with T. Ozaki, M. W. Rupich, L. Wu, W. Si, and V. Solovyov.

The critical current density (J_c) in High Temperature Superconductor (HTS) such as Y₁Ba₂Cu₃O_7-x (YBCO) can be enhanced by introducing defects into the material. These defects can act as pinning centers to restrict the motion of magnetic flux vortices, which as a result increases the current carrying capacity of a superconducting material. One method to create vortex pinning centers in the material is to introduce nanodefects during the synthesis process. Another is to use a post-synthesis ion beam irradiation technique to further induced defects onto an existing defect landscape within HTS to further enhance J_c. Particle irradiation provides a high precision method to optimize the defect landscape for critical current. We demonstrate that proton and oxygen beam irradiation can double the J_c of YBCO coated conductors with existing nano precipitates in fields of 6 Tesla parallel to the c-axis at T=27 K. In this work, HTS coated conductors containing Barium zirconate nanorods as pre-existing defects were irradiated with 50 MeV copper ions at angles of 0^o, 15^oand 30^o from the crystallographic c-axis. We observed moderate enhancement of J_c at 5 K at high fields in samples irradiated at 30^o and a suppression in others.

With the advent of all-superconducting very-high field (>20T) magnets, it becomes technologically relevant to study superconducting critical currents in magnetic fields in the range of 20 to 65 T. New challenges to vortex pinning understanding may also emerge at such very-high fields and low temperatures. For example, are thermal fluctuations as important at high-fields/low-temperatures as they are at liquid Nitrogen temperatures near the irreversibility line?
We present critical current measurements obtained by electrical transport measurements in pulsed magnetic fields up to 65T. The use of Fast Programmable Gate Array (FPGA) facilitates fast measurements/decision times permitting multiple current-voltage (I-V) curves measurements in a single magnetic field pulse lasting 50ms.
We study and compare standard YGdBa₂Cu₃O₇ coated conductors with and without BaHfO₃ nanoparticles grown by Metal Organic Deposition. These films have shown great results in terms of critical current enhancement up to 9T at high and low temperatures owing to the small size of BaHfO₃ nanoparticles. Our results show continuous improvement for samples with BaHfO₃ and will be analyzed in detail in the presentation.

It is well known that YBa₂Cu₃O_7-x high temperature superconductor bulks can carry transport current in high magnetic field for many practical engineering applications. In this paper, three-dimensional YBCO compounts with designed shapes, high density and excellent superconducting properties were first successfully prepared by an extrusion-based additive manufacturing or 3D printing technique involving aqueous and non-aqueous pastes composed of milled precursors. The obtained milled powder can effectively reduce sintering temperature and simultaneously improve densification of YBCO compared with unmilled sample. Then, milled powder dispersed in aqueous and non-aqueous systems were printable to build YBCO part layer by layer, exhibiting shear thinning behaviors with the viscosity, density and porosity mainly related to their binder type and solid loading. It was found that non-aqueous PVB solution and high solid loading seemed to have a slightly positive effect on density, reaching up to 93% of theoretical density after just sintering at 940 ^oC for 10 h. To the end, a printed YBCO hollow cylinder with large trapped magnetic field demonstrated its ability of prolonging the levitation time, extending the potential application in flywheel technology. The integration of extrusion printing technique with milled precursors realizes the possibility of producing complex-shaped and scale-up YBCO with useful electromagnetic properties for a broad range of applications.

Low temperature superconductors like Nb-Ti and Nb₃Sn have enabled virtually all of today’s superconducting magnets including machines like the Large Hadron Collider with 8 T Nb-Ti dipole magnets and 1 GHz NMR spectrometers with 23 T field supplied by hybrid Nb-Ti/Nb₃Sn magnets, presently the highest field all-superconducting user magnet. A recent NRC report [1] has described the scientific and technology rationales for many new types of ultra-high field superconducting magnet: regional 32 T superconducting magnets, a 40 T superconducting magnet, 28 – 37 T high-resolution NMR superconducting magnets, 25 – 40 T for x-rays and neutrons, 60 T DC, 20 T for human MRI, as well as magnets for fusion, particle-accelerators, radiotherapy, axion and other particle detectors. The materials and magnet technology have now made sufficient progress that a few of these magnets are now feasible. I will describe this progress and the various pluses and minuses of the three present high-temperature superconductor types and suggest some timelines under which high field coils beyond the capabilities of present-day Nb-Ti and Nb₃Sn magnets might start to make it into commercial fabrication. At the NHMFL we have shown that High Temperature Cuprate Superconductors can generate test coils with fields of 45 T, almost twice the Nb-based superconductor limit and now equal to the world record 45 T DC field of the NHMFL User Magnet which requires 28 MW of DC power and a large 11 T Nb₃Sn superconducting outsert magnet.

*Work carried out with many collaborators at the MagLab, especially with programs led by Huub Weijers, Denis Markiewicz (32 T), Ulf Trociewitz (Bi-2212 coils), Seungyong Hahn (NI REBCO), Scott Marshall (Bi-2223), Eric Hellstrom, Jianyi Jiang, and Fumitake Kametani (Bi-2212 conductors), and Dmytro Abraimov and Jan Jaroszynski REBCO conductors.

[1] B. I. Halperin BI et al., High magnetic field science and its application in the United States: Current status and future directions. Washington, D.C.: The National Academies Press, 2013. http://www.nap.edu/catalog/11211/opportunities-in-high-magnetic-field-science.


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The proposed Future Circular Collider at CERN should reach a collision energy of 100 TeV thanks to a four times larger radius and double the magnetic field of the LHC. The latter requirement necessitates the development of superconducting materials/conductors with increased operating field.
Whilst most efforts are focused on Nb₃Sn wires, this is an opportunity to assess the potential for high field application of newer superconductors. In the context of a collaboration between CNR-SPIN and CERN, we plan to investigate three different superconducting materials, i.e. Bi-2212, MgB₂ and iron-based superconductors (IBS), with the scope of advancing their performance using industrially scalable production methods.
MgB₂ conductors can be realised by the powder in tube (PIT) method and have the advantages of low cost and a relatively high critical temperature, T_c. However, the upper critical field, H_c2, and pinning in these conductors is still not optimised. Through the development of a boron precursor synthesis route, we plan to produce MgB₂ nanopowders with the inclusion of controlled defects.
Bi-2212 wires realised by the PIT method have shown good performance at high fields. However, these results required heat treatment under pressure that is incompatible with the production of large coils for magnets. We plan to approach the performance today obtained by high pressure heat treatment through mechanical deformation and heat treatment sequences, including optimisation of the temperature profile and oxygenation conditions.
Recently discovered IBS exhibit high T_c and huge H_c2. PIT and coated conductor tapes have been successfully realised, with critical current values exceeding the threshold for practical application (10⁵ A/cm² at 10 T). We plan to develop prototype IBS conductors that achieve this critical current density at 16 T through reliable, simpler and scalable techniques that would permit industrialisation.
In this talk, the main goals and methods that we are developing will be reviewed.

It’s been over 30 years since the discovery of high-temperature superconductivity, but the finicky intrinsic properties of these game-changing materials have required countless scientists and engineers to hone into a form that can now be applied as a true conductor technology on a macroscopic scale. Coated conductors, in which ReBCO thin films are deposited on a technical substrate utilizing a number of complex industrial processes, are finally becoming available from several vendors in long-lengths at costs that are likely to decrease in the coming years. The form-factor of coated conductors, an anisotropic tape with a high aspect-ratio, still presents a challenge in terms of cabling and winding into practical magnet systems. Studies on the strain-state within the ReBCO layer as a function of bending led to the development of the Conductor-on-Round-Core (CORC^®) concept, in which several ReBCO coated conductors are wound helically around a relatively small former. These round, isotropic, HTS conductors offer a wide variety of operating currents and current densities to suit a range of applications. CORC^® cables with thickness of 5 to 8 mm have been developed for power transmission and for use in large magnets, that require only very limited bending of the cable, or bundled into 6-around-1 cable in conduit conductors (CICC) for fusion and detector magnets. Much more flexible CORC^® magnet wires with thickness of 2 to 4 mm are being developed for solenoids and accelerator magnets that require a current density (J_e) of at least 300-600 A/mm² at 20 T. An overview of the current status and future developments of CORC^® cables and wires is presented here with an emphasis on conductor processing and application.

Acknowledgement
This work was in part supported by the US Department of Energy under agreement numbers DE-SC0007891, DE-SC0007660, DE-SC0009545, DE-SC0014009 and DE-SC0015775 and the U.S. Navy under contract numbers N00024-14-C-4065 and N00024-16-P-4071

Bi-2212 is the only cuprate superconductor that can be made as a round wire. It has high J_c and H_c2, can be fabricated with a variety of architectures, can be twisted and fully transposed as a Rutherford or a 6-on-1 cable, and can be lightly aspected to form a square or rectangular conductor. These characteristics are advantageous to design and build high-field magnets. Bi-2212 powder is now available from two sources, MetaMateria and nGimat, and short samples of wire made with nGimat powder have J_c = 9530 A/mm² (4.2 K, 5 T) and J_c = 6860 A/mm² (4.2 K, 15 T) with overpressure processing. Bruker-OST is fabricating > 1.4 km long Bi-2212 wire piece lengths with these powders. We have designed, modelled, wound, heat treated and tested coils that are I_c and stress/strain limited. There is good agreement between predictions from the models and the measured performance of the coils. This presentation will highlight the improvements in powders, wires, and heat treatments that underlie these advances in the performance of Bi-2212 wire.

Achieving high J_c in Bi₂Sr₂CaCu₂O_x (Bi-2212) round wire requires overpressure heat treatment (OPHT) to eliminate current-limiting bubbles. The OPHT has a maximum temperature of 890 °C and is done with 50 bar overpressure with 1 bar oxygen partial pressure (PO₂). During OPHT, the diameter of the Bi-2212 wire shrinks about 4%, with about 80% of this densification occurring during a 2 h hold at 820 °C before the Bi-2212 powder melts. As part of his PhD studies, Maxime Matras from our group investigated predensifying Bi-2212 round wire for 2 h at 820 °C at 50 bar overpressure with different PO₂, followed by the full OPHT with 1 bar PO₂. A surprising result of his study was that J_c of Ag-0.2 wt% Mg sheathed Bi-2212 round wire increased by more than 20% when the 820 °C predensification was done in 5 bar PO₂, compared to predensification using 1 bar PO₂. We are investigating what causes this significant increase in J_c and the processing parameters (t, T, PO₂) to optimize this increase in J_c by predensification. This presentation will report on results of on-going predensification studies at temperatures ranging from 500 °C to 850 °C, at different PO₂, for different times. J_c, irreversibility field, and critical temperature are being measured to determine the optimum heat treatment parameters. Scanning electron microscopy of fully-processed Bi-2212 wire is being used to understand the microstructure-property relationships.

To meet J_c requirements for the Future Circular Collider (FCC) the high-field performance of Nb₃Sn strands must improve. In the last decade the best in-field performance of Nb₃Sn strands has been obtained by doping with Ti or Ta, however, these wires are far from homogeneous in phase or properties. It has been demonstrated that Ti doping can produce a more chemically homogeneous A15 layer in internal Sn wires and that it significantly narrows the distribution of properties (T_c, H_c2) [1]. However this occurs in a still significantly suppressed stoichiometry (23.1 and 23.4 at.%Sn in Ti and Ta doped wires, respectively). Moreover, the effects of doping is not fully understood: early studies suggested that both dopants preferentially substitute on the Nb site. But more recently, taking into consideration ternary compositions and the different effectiveness of Ti (1-2 at%) and Ta (3-4 at%) in changing the resistivity and obtaining the maximum H_c2, it was suggested that Ti might substitute on the Sn site. In order to resolve this conundrum and point to new ways to improve the Nb₃Sn performance, we recently performed an Extended X-ray Absorption Fine Structure (EXAFS) study at the APS synchrotron in Argonne [2]. The EXAFS technique is sensitive to the local environment of a specific element in the structure and so it allows us to identify the site location of the dopants in modern high-performance Nb₃Sn strands. We investigated Ti, Ta and Ta+Ti doped internal Sn wires and we found that, whereas Ti exclusively occupies the Nb site in the A15 structure, Ta unexpectedly occupies both sites. In particular we have determined that about 21-32% of Ta resides on the Sn site in the Ta-doped and the Ta+Ti doped wires. With such occupancies the (Nb+Ti):Sn ratio becomes strongly off-stoichiometry (~3.3-3.4) whereas the Ta and Ta+Ti doped samples become almost stoichiometric (~2.9-3.1), thus we have the paradox that the Ti-wires, which have the best performance of accelerator Nb₃Sn wires, are the most off-stoichiometric.
Combining those findings with magnetic and specific heat characterization, we will discuss how the different occupancy affects the disorder and, as a consequence, the H_c2 behavior and the effect of charge doping in changing the A15 phase DOS. We will also discuss how this understanding could help to redesign an improved internal Sn strand for higher field application such as the 16T FCC project.
[1] C. Tarantini et al., Appl. Phys. Lett. 108, 042603
[2] S. M. Heald, C. Tarantini et al., in preparation.
Acknowledgements
APS supports: the US DOE-BES, the CLS and its funding partners, the Advanced Photon Source, the US DOE under Contract No. DE-AC02-06CH11357. NHMFL partial supports: the US DOE-SC, HEP under Award No. DE-SC0012083 and the NHMFL supported by NSF Coop. Agr. No. DMR-1157490 and the State of Florida.